Cast aluminum and cast iron engine blocks typically have crankshaft bearing blocks spaced axially along the engine at locations for a front main bearing, a rear main bearing, and a plurality of intermediate main bearings. After preparatory machining operations on the block, for finish machining, bearing caps are installed on the bearing blocks so that the bearing blocks and bearing caps are machined in situ to provide a precision crankshaft bearing bore to a defined roundness and axial alignment. Because the bearing blocks are spaced apart, this configuration is sometimes referred to as a bearing ladder. Most engine blocks also typically have a series of camshaft bearing blocks spaced axially of the engine at locations for front and rear end bearings and one or more intermediate bearings, also sometimes referred to as a bearing ladder. Of course, for overhead camshaft engines, one or more camshaft bearing ladders are in the cylinder heads.
Crankshaft and camshaft bearing ladders may be machined by either reaming or boring. A line boring bar typically used for back boring has radially-projecting, generally axially-aligned cutting tools spaced axially along the bar at locations to simultaneously back bore respective bearing block bores. Due to the length of an engine block or a cylinder head, a relatively long boring bar is required, say a minimum of about 24 inches. The length of the boring bar and the severity of the machining forces typically require an outboard bushing to support the free or distal end of the boring bar during back boring. Generally inboard and/or intermediate support bushings are used to minimize tool chatter. Because the radius through the cutting tools exceeds the bore radius, the boring bar must be inserted through the bore with the boring bar axis offset from the bearing bore axis. After the cutting tools have cleared the bearing blocks, the boring bar axis can then be aligned coaxially with the bearing bore axis for the back boring operation.
Prior to the present invention, applicant's assignee has used single-axis boring machines wherein the spindle moves only on its rotational axis to insert and retract the boring bar. With the spindle retracted, a lift-and-carry transfer brings the engine block into the work station and locates the block in an initial setup position, with the bearing bore axis offset vertically from the boring bar axis to provide clearance in a vertical direction for the cutting tools. With the axes offset, the spindle can then be rapidly extended to insert the boring bar through the bearing bore. The block is then lowered or dropped onto precision locators and clamped in place in a final machining position with the boring bar axis coaxial with the bearing bore axis. Other arrangements such as a machining fixture with retractable setup plungers can also be used. After a relatively short back boring stroke of a few inches, the block is raised slightly by the transfer and/or setup plungers to provide a clearance offset between the bore and boring bar axes and the spindle is then rapidly retracted to withdraw the boring bar. Two opposed single-axis boring machines as well as two single-axis machines at the same side of the block have also been used so that camshaft and crankshaft bores can be machined at the same work station.
Similar prior art back boring stations are described in U.S. Pat. No. 5,221,165 granted Jun. 22, 1993 and U.S. Pat. No. 4,693,642 (Mair et al) granted Sep. 15, 1987. Such boring stations operate satisfactorily and are relatively cost efficient for long manufacturing runs of a single engine type, for example for a given V-8 engine. However, different engine types such as cast iron, cast aluminum, V-8, V-6, V-10, quad, etc., have different camshaft and crankshaft locations and typically require different tools and, in the case of a line boring bar, different clearance offsets to insert and retract the boring bars. Consequently, when production is switched to a different engine type, substantial down time is required to set up single-axis boring machines, e.g., precision setup for locating, clamping, transfer and the like. Alternatively, different stations must be used for different engines, substantially increasing the initial capital investment, not only for duplicating the stations but for additional conveyers, transfers and the like, and more floor space is required. With a single-axis boring machine and an in-line boring bar, this setup is aggravated by variations in the vertical location of the bores on the part; that is, the new part, such as an engine block, must be relocated relative to the boring bar axis, for both offset and coaxial alignment. This may even make it impractical to do both crankshaft and camshaft bores on more than one engine type without relocating and reclamping the part. Relocation and reclamping complicates precision locating of the camshaft bore to the crankshaft bore, increases machine cycle, set up and part transfer times.
The Goszczynski U.S. Pat. No. 5,221,165 and Mair et al U.S. Pat. No. 4,693,642 patents further delineate some of the drawbacks of the above described prior art boring method and apparatus and describe alternative boring methods and boring bars. Both alternatives use special boring bars with cutting tools on the distal end remote from the spindle, i.e., a single-point cutting tool or a multiple cutter assembly. In both alternatives the boring bars have special bearing structures or bearing pads which support the boring bar in the journals as the boring bar is advanced into the workpiece. This latter type of boring bar does have certain advantages over back boring using a line boring bar for certain applications, even though the tooling is special. However, back boring bars are still often used commercially for a variety of reasons, e.g., the boring bar is in tension and less likely to flex during machining and the back boring bar may be a conventional and simpler design.
The introduction of the "just-in-time" concept and "flexible" manufacturing systems in automobile manufacturing and the need for close and consistent tolerances, coupled with a continuing need to more effectively utilize engine lines to machine different engine types, presents a special need to eliminate disadvantages of the prior art boring stations.